This invention relates generally to magnetic recording media, and more particularly to a magnetic recording disk with an antiferromagnetically-coupled (AFC) magnetic recording layer of the type described in the above-cited co-pending application.
Conventional magnetic recording media, such as the magnetic recording disks in hard disk drives, typically use a granular ferromagnetic layer, such as a sputter-deposited cobalt-platinum (CoPt) alloy, as the recording medium. Each magnetized domain in the magnetic layer is comprised of many small magnetic grains. The transitions between magnetized domains represent the xe2x80x9cbitsxe2x80x9d of the recorded data. IBM""s U.S. Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventional rigid disk.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (where Mr is measured in units of magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (Hc) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hc. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the stored magnetic information in the layer will be more likely to decay. This decay of the magnetization has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions.
One approach to the solution of this problem is to move to a higher anisotropy material (higher Ku). However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Ms (Ms=saturation magnetization), becomes too great to be written by a conventional recording head. A similar approach is to reduce the Ms of the magnetic layer for a fixed layer thickness, which will reduce Mr since Mr is related to Ms, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
IBM""s previously cited co-application describes a magnetic recording medium wherein the magnetic recording layer is at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic spacer film. In this type of magnetic media, referred to as AFC media, the magnetic moments of the two antiferromagnetically-coupled films are oriented antiparallel, with the result that the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in volume V. Therefore the thermal stability of the recording medium is not reduced. In one embodiment of the AFC medium the ferromagnetic films are sputter deposited CoPtCrB alloy films separated by a Ru spacer film that has a thickness to maximize the antiferromagnetic coupling between the two CoPtCrB films. One of the ferromagnetic films is made thicker than the other, but the thicknesses are chosen so that the net moment in zero applied magnetic field is low, but nonzero.
AFC media is to be distinguished from conventional xe2x80x9claminatedxe2x80x9d media, wherein two or more magnetic layers are spaced apart by an nonmagnetic spacer layer so that the magnetic layers are deliberately magnetically decoupled. It is known that substantially improved SNR can be achieved by the use of laminated media. The reduction in intrinsic media noise by lamination is believed due to a decoupling of the magnetic interaction or exchange coupling between the magnetic layers in the laminate. This discovery was made by S. E. Lambert, et al., xe2x80x9cReduction of Media Noise in Thin Film Metal Media by Laminationxe2x80x9d, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2706-2709, and subsequently patented in IBM""s U.S. Pat. No. 5,051,288. IBM""s more recent U.S. Pat. No. 6,077,586 describes a laminated disk with special seed layers and magnetically decoupled boron-containing ferromagnetic layers.
In general, adding more layers to a disk structure adds complexity to the manufacturing process. Because AFC media require a larger number of sputtering stations due to the larger number of layers that must be sputtered, it may be necessary to substantially modify the existing disk manufacturing line. In addition, the use of a boron-containing alloy like CoPtCrB in the AFC films, which is necessary for high performance media, requires the use of a special onset or nucleation layer to enhance the growth of the CoPtCrB films so that the C-axis of these films is in the plane of the films. The nucleation layer, which is typically a nonferromagnetic CoCr alloy, requires still another sputtering station in the manufacturing line.
What is needed is AFC media that possesses high performance magnetic properties but does not require the addition of sputtering stations to the existing manufacturing line.
The invention is an AFC disk wherein the lower ferromagnetic film in the AFC recording layer is a boron-free ferromagnetic CoCr alloy that does not require a nucleation layer between it and the Cr or Cr alloy underlayer. The ferromagnetic CoCr alloy has sufficient saturation magnetization (Ms) and grain structure to produce excellent magnetic recording performance for the AFC recording layer, while also serving as a nucleation layer to induce the in-plane C-axis growth of the top boron-containing ferromagnetic film through the spacer layer.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.